Significant use of dialysis therapy for patients suffering from renal disease has only been realized in recent years and the use of clinical chemistries to measure the progress of dialysis therapy is infrequent with end stage renal patients since the loss of blood for analysis must be held to a minimum in these patients because of chronically low hemocrits and because the cost of serum screens is high due to the operation of automated clinical analyzers for multiple factor analyses. As a consequence, the progress of most end stage renal patients on dialysis is followed only by a monthly assay for serum levels and by body mass measurement at each treatment session. However, it is generally recognized that a simple assay such as blood urea nitrogen (B.U.N.) would provide invaluable information on the progress of the therapy thereby enabling monitoring of the therapy and providing the opportunity for improved control of therapy. With prior art apparatus and methods such monitoring and control is accomplished by sampling the blood of the patient, and as noted previously, such techniques are not satisfactory.